avcenc: Prepend startcodes to output buffers

This makes the encoder output data that is similar to what
actual HW encoders output. This form is also what is expected
by virtually every other component - the only exception
seems to be stagefright's MPEG4Writer which happens to work
with this format.

This also makes the encoder's output work in other decoders
when passing through directly. If feeding the encoded data
straight through to a decoder (such as in the EncodeDecodeTest
CTS test), the output from OMX.google.h264.encoder cannot be
decoded by e.g. OMX.Nvidia.h264.decode on a Nexus 7 or
OMX.SEC.avc.dec on a Galaxy S3.

Change-Id: I6e16433a4a66936e3080723c8233681f53bd55e5
avcenc: Only do startcode escaping if the next byte requires it

Section 7.4.1 in the H.264 standard says that the only valid bytes
to follow a sequence that starts with 0x000003 are 0x00, 0x01,
0x02 or 0x03.

This makes EncodeDecodeTest pass properly when decoding using
OMX.google.h264.decoder, which is strict about the forbidden
escape sequences.

Change-Id: Ice113d9b934015003ea9cb10d0b21cee4d18d774